I think there are like three issues here.
1. Should the alias be removed? Yes, because it's not technically a 1:1 mapping. There are non-zero cases of girls with dicks that have a more specific tag than futanari (right now).
2. Is this the correct way to view this set of anatomical changes? The issues around newhalf seem to stem from the idea that indications of transition are required (either in-canon or TWYS rules). In a way, having a dickgirl tag as the "noun" and modifiers around surgery? hormone treatments? in-canon transition? could support that.
There is merit to being specific, because it helps people find the images they want. If the classification system is too obscure, then there's little merit because it's not going to be used and become a gardening problem. I haven't seen a quantitative way of determining whether or not specificity for a given tag outweighs the gardening overhead, but it would be damn useful.
3. TWYS strictness? I don't think it's worth being overly strict. It's good advice, but once a character "obviously" falls into a bucket that's not seen, it'll be all fuzzy again. TWYS should be strict when the tags are purely about visual things. But if we're going to have tags that are more philosophical then it shouldn't be a rule to live/die by. We have a lot of very specific cum location tags, so we could be more specific if necessary for newhalf.